In the aforementioned fields of application, the transmission of digital data with a high degree of reliability and security comes up against a major obstacle, that of the transmission of these data by way of a variable transmission channel whose characteristics are not known a priori. The digital data transmitted are subdivided into symbols, consisting of strings of bits of these data, each symbol allowing the modulation of a carrier radio wave transmitted over the channel.
The very strong demand for reliable high bit rate radiofrequency transmission processes has brought about the initiation and execution of numerous research projects relating to the definition and implementation of future-generation TDMA (Time Division Multiple Access) mobile radio communication systems.
Radiofrequency transmission channels are known by the fact that they are both frequency selective and time varying. The temporal variation is consequent upon the mobility or the speed of the user or users. Their frequency selectivity results from the conditions of propagation of the radiofrequency signals via multiple paths and the destructive superposition of the signals received, arising from propagations over these various paths. The phenomenon of frequency selectivity brings about a phenomenon of intersymbol interference, prejudicial to the quality of transmission and detection of these symbols upon reception thereof. The phenomenon of intersymbol interference and the complexity of the receivers are substantially heightened with the transmission bit rate.
These specific characteristics of the above-cited radiofrequency transmission channels have always led to the implementation of particularly subtle tailored radiofrequency interfacing systems, all the more so when high bit rate transmission with high spectral efficiency is sought.
Nevertheless, the aforesaid frequency selectivity and temporal variation, regarded a priori as major obstacles, of radiofrequency transmission channels have however been the subject hitherto of investigations, by way of the concept of diversity, as will be explained hereinbelow.
In this respect, the concept of turbo-code presented by C. BERROU, A. GLAVIEUX, P. THITIMAJSHIMA in the article entitled “Near Shannon limit error correcting coding and decoding: Turbo Codes”, IEEE ICC'93, pp. 1064–1070 Geneva, Switzerland, May 1993, has rekindled interest in iterative processes both from the theoretical point of view and from the practical point of view.
The remarkable success of such a concept resides in three of its specific aspects: quasi-random nature, concatenation of several compounded codes of low complexity, and iterative decoding by weighted input/output of each constituent code, by virtue of the use of information available from all the other codes.
A generalization of these concepts has led to a novel approach, dubbed the principle of turbo-detection, within the field of communication theory, this approach consisting of a recursive updating of random a posteriori information on data or symbols from among the set of functions concatenated in the reception chain.
The turbo-detection process described in the article published by C. DOUILLART, M. JEZECHIEL, C. BERROU, A. PICART, P. DIDIER, A. GLAVIEUX and entitled “Interactive Correction of Intersymbol Interface: Turbo-Equalization” published in European Trans. On Telecommunication, Vol. 6, pp. 507–511, September 1995, would appear to be a highly beneficial application of the “turbo-detection principle” concept, aimed at reducing or inhibiting the intersymbol interference phenomena generated by a radiofrequency transmission channel.
By modelling the structure of intersymbol interference as a nonsystematic nonrecursive and time-varying convolutional code of unit rate, the detection of data and the decoding (equalization) of the transmission channel may be identified formally with a serial concatenation of two trellis codes. The maximum-likelihood optimal decoding of the set thus formed, conditioned upon the perfect knowledge of the transmission channel, can then be achieved by virtue of an iterative process similar to that described by S. BENEDETTO, D. DIVSALAR, C. MONTORSI, F. POLLARA, in the article entitled “Serial Concatenation of Interleaved Codes: Performance Analysis, Design and Iterative Decoding” and published by TDA Progress Report 42–126, August 1996.
Various fields exhibiting some interest have appeared in the wake of earlier studies relating to turbo-detection.
One of these fields relates to the estimation of a mismatched channel, such as described in the article by G. BAUCH, V. FRANZ, entitled “A Comparison of Soft-in Soft-out Algorithm for Turbo-Detection” and published by Proceedings of ICT, Vol. 2, pp. 259–263, Portos Carras, Greece, June 1998 and which has been at least partially solved through the implementation of a “fully turbo” receiver developed by A. O. BERTHET, B. SAYRAC ÜNAL, R. VISOZ. The basic concept consists in superimposing on the architecture of the turbo-detector an iterative process of channel reestimation, which exploits the information available on the symbols after channel decoding.
Another of these fields subjected to investigation consists in strengthening the inner code relating to the intersymbol interference phenomenon through the introduction of a trellis coded modulation (TCM), as has been proposed by R. VISOZ, P. TORTELIER and A. O. BERTHET in the article entitled “Generalised Viterbi Algorithm for Trellis coded Signals transmitted through Broadband Wireless Channels” published by Electronic Letters, pp. 227 to 228, 3 Feb. 2000, Vol. 36, No. 3 and by A. O. BERTHET, R. VISOZ, B. ÜNAL and P. TORTELIER in the article entitled “A Comparison of Several Strategies for Iteratively Decoding Serially concatenated Convolutional Codes in Multipath Rayleigh Fading Environment” published by Proc. IEEE GLOBECOM' 2000, San Francisco USA, November 2000. In the latter article, a serially concatenated TCM code scheme has turned out to provide at least two advantages according to which:    1. the decoding can begin sooner, by comparison with the conventional turbo-detection process, while the performance is asymptotically better;    2. the computational complexity can be reduced by carrying out a SISO detection of the data and a joint TCM decoding on the only reduced-state TCM trellis.
However, by its very nature the turbo-detection process fully utilizes the diversity introduced by the coding and the interleaving, and, consequently, its performance is strongly related to the depth of interleaving.
Although the process turns out to be efficient even with regard to the worse configurations of static intersymbol interference, it cannot be specially tailored to specific radiofrequency profiles, cf. ETSI.GSM Recommendations, 05.05 version 5.8.0 December 1996, where most of the outputs of the channel are in general easy to equalize but characterized by deep frequency fading.
When the transmission channel disturbance generated by the variation in the temporal distribution of energy supplants the frequency-selective dispersion, the turbo-detection process remains ineffectual, in particular in the case of delay-sensitive applications. Cf. the article by M. PUKKILA “Turbo Equalisation for the Enhanced GPRS System” published by IEEE conf. PIMRCOO London, UK, 2000. This is why, with the aim of achieving the best possible performance, advanced TDMA mobile systems must be designed both to combat the phenomenon of intersymbol interference and to cater for other forms of diversity, that is to say antenna spatial diversity.
To benefit from the phenomenon of spatial diversity, by virtue of the spatio-temporal coding techniques, as described by V. TAROCK, N. SESHADRI, A. R. CALDERBANK in the article entitled “Space Time Codes for High Data Rata Wireless Communication: Performance Criterion and Code Construction” published by IEEE Trans. Inform. Theory. Vol.44, No. 2, March 1998, while implementing turbo-detection, the basic communication model proposed implements an outer code, essential for the turbo-detection process, interleaved with a spatio-temporal trellis coded modulation (ST-TCM).
In fact, such a model must be regarded as a serially concatenated spatio-temporal trellis coded modulation. It makes it possible to maintain the essential advantage consisting in allowing joint equalization and inner spatio-temporal decoding by virtue of sub-optimal SISO algorithms of reduced complexity, contrary to the distinct more complex approach, described by C. BAUCH, A. NAGUILS, N. SESHADRI in the article entitled “PA Equalisation of Space-Time Coded Signals over Frequency Selective Channels” published by Proc. Wireless Communications on Arraying Conference (WCNC) September 1999, according to which approach the data detection and the spatio-temporal decoding are carried out separately in an iterative manner.
Finally, various projects aimed at affording a significant improvement to the spectral efficiency of codes of the serially concatenated spatio-temporal trellis coded modulation type, hereinbelow designated serially concatenated ST-TCM codes, have been published.
To the knowledge of the inventors, there are at most four distinct approaches which can allow an improvement in the spectral efficiency of serially concatenated ST- TCM codes:                A first possibility consists in reducing the coding rate to the maximum, both of the inner code and of the outer code.        Unfortunately, a consequence of reducing the coding bit rate of this outer code is poor performance of the turbo-detection.        A second possibility consists in increasing the order of modulation of the serially concatenated ST-TCM code.        Such an increase, however, beyond an order 4, through the implementation of better configurations of the best known ST-TCM codes, such as those described by V. TAROCK, N. SESHADRI, A. R. CALDERBANK in the above-cited article, results in a very great reduction in the performance of the inner code, which code reduces to a combination of the ST-TCM code and of the channel code.        By reason of the fact, however, that the serially concatenated TCM code schemes consisting of simple QPSK modulations coded by a convolutional code of rate ½ have proved to be very efficient within the framework of numerous intersymbol interference environments, as described by A. O. BERTHET, R. VISOZ, B. ÜNAL and P. TORTELIER in the article entitled “A comparison of Several Strategies for Iteratively Decoding Serially Concatenated Convolutional Codes in Multipath Rayleigh Fading Environment” cited above, the third possibility consists in multiplying the digital data of several users, or equivalently, various distinct data streams, in the same time interval of the TDMA system. Such an approach makes it possible to increase the overall spectral efficiency of the system. Within the framework of this third possibility, a first implementation can consist in modelling a multi-user communication by multiplexing several fully independent serially concatenated ST-TCM coding processes and by regarding each distinct input data stream as a specific user. Such an implementation does not however utilize the phenomenon of spatial diversity.        A fourth approach consists finally in demultiplexing a single pre-encoded data stream on a plurality of transmission antennas, in accordance with the BLAST process (Bell Labs Layered Space-Time) as described by G. J. FOSHINI, G. D. GOLDEM, R. A. VALENZUELA, P.W. WOLANIANSKY, in the article entitled “Simplified processing for High Spectral Efficiency Wireless Communication Employing Multi-element Arrays” published by IEEE JSAC, Vol. 17, No. 11, pp. 1841–1852, November 1999.        
In the last two approaches, the radiofrequency interface thus described is in particular based on the use of several antennas in transmission and in reception and relies, in order to attain very high bit rates and high spectral efficiency, on the parallel transmission of several data streams coded by a spatio-temporal code, STC code, corresponding substantially to an ST-TCM code.
In particular, the technique allowing the implementation of the aforesaid radiofrequency interface exhibits the major drawback of not supporting intersymbol interference, by reason of the use of a suboptimal linear receiver whose performance is therefore, in the absence of coding, related directly to the rank of the transfer matrix representative of the transmission channel, the phenomenon of spatial diversity on transmission and on reception was alone taken into account.
Furthermore, owing to the linear processing alone introduced, the number of antennas in reception cannot be less than the number of antennas in transmission, the number of antennas in reception even having to be increased, in relation to the number of antennas in transmission, so as to try to improve the performance and the level of quality of detection and of reception although the test transmission channel was a channel devoid of intersymbol interference. In conclusion, the performance of such a radiofrequency interface remains tightly related to the propagation conditions existing on the transmission channel and direct application of such a radiofrequency interface to mobile radio telephony cannot readily be envisaged, since mobile radio telephony receivers, by reason of their size and their reduced bulk, scarcely admit a number of transmission/reception antennas greater than two.